Surface modified membranes for gas separation, and a method for preparing thereof

ABSTRACT

The present invention relates to a CO2 selective gas separation membrane and a method for preparing the gas separation membrane and the use thereof. The CO2 selective gas separation membrane comprises a gas permeable or porous support layer; and at least one gas permeable polymer layer, which is surface modified with polymer chains having CO2 philic groups, wherein the gas permeable polymer layer has a spatially controlled distribution of the CO2 philic groups on the surface thereof. The method of preparing the CO2 selective gas separation membrane, comprises the steps of: depositing at least one gas permeable polymer layer on a porous or gas permeable support layer to form a dense membrane, and surface modifying the dense membrane with polymer chains having CO2 philic groups, to obtain spatially controlled distribution of the CO2 philic groups on the surface thereof.

TECHNICAL FIELD

The present invention relates to CO₂ capture from gas mixtures by use ofgas separation membranes. In particular, the invention relates to CO₂selective polymeric membranes and the method of producing suchmembranes. The invention is also directed to the use of the CO₂selective polymeric membranes.

BACKGROUND/PRIOR ART

Existent technologies for CO₂ capture from flue gas streams, such asconventional absorption or adsorption, have a high energy consumptionand overall costs and represent a major obstacle for industrialimplementation in the following major markets (by amount of CO₂emitted): energy sector (flue gas from power plants), oil and gas(natural gas sweetening), industry (flue gas from e.g. cement,steelmaking). In addition, the chemicals used in amine absorptionrepresent an extra pollution source to the environment.

Membrane technology for gas separation has become widely used. Whilepolymeric membranes are economical and technologically useful, they arelimited by their performance. The balance between permeability andselectivity affects the use of polymeric membranes for CO₂ separationfrom flue gas streams, and CO₂ separation becomes very expensive due tolow permeability, which will require an extremely big membrane arealeading to high investments costs.

In membrane science it is assumed that in order to selectively separatetwo gases in a gas mixture you need to prepare a dense, selective(towards one of the gases) polymer layer which is either:

1) uniform (made of same polymer material)2) a blend of different polymers, or3) a mixture of polymer(s) with particles.

The dense polymers layers may be CO₂ selective because of the intrinsicchemical structure of polymer; the polymers have CO₂ philic groups, suchas amines, in their polymer chains, and/or they have added particles(carbon nanotubes, silica, zeolites, etc.) to enhance the CO₂selectivity or permeability of a given polymer.

In order to form a thin layer (i.e. in the range 200 nm to 100 μm), theuniform polymer (1), the blend of different polymers (2), or the mixtureof polymers with particles (3) are mixed in a solution with a solventand casted/coated/deposited as a film that evaporates and forms a denseselective membrane. The composition of the polymer layers of themembrane and the spatial arrangement/alignment (vertical or horizontal)of the CO₂ philic groups or particles relative to direction of gasmolecules from the feed side of the membrane to the permeate side arerelatively random and depend inter alia on the mixing of the components,compatibility of polymer and CO₂ philic entities (chemical groups,particles or other polymer); gravity, how fast solvents evaporates,particle conglomeration, etc.

Polymeric membranes separate the CO₂ from a large and dilute stream(˜1-20% CO₂) due to higher CO₂ solubility and/or diffusion coefficient(solution-diffusion mechanism) compared to other gases such as N₂ and O₂(flue gas, breathing), CH₄ (natural gas, biogas), or H₂ (syngas). TheCO₂ selectivity versus the other gases (N₂, O₂, CH₄, H₂ or other gases)and CO₂ permeability is given by intrinsic properties of the membranematerial. The driving force for the transport of gas molecules through amembrane is due to a partial pressure or concentration differencebetween feed and permeate side created by using a sweep gas or vacuum onthe permeate side of the membrane.

One option for membrane separation is the use of a facilitated transportmembrane. The most known membrane type using facilitated transport issupported liquid membrane (SLM) with mobile facilitated transportcarriers. These have been studied for over two decades and are known tohave both high gas permeability and high gas selectivity. However, forthe SLM membranes serious degradation problems, such as evaporation andleakage from membrane of solution and deactivation of the carriers haverestricted their further development and application.

Gas separation membranes still have a need for improved CO₂ separationperformance in order to be cost effective for industrial applicationsespecially at low CO₂ concentrations in a mixture, below 20%,(preferably below 10%, below 5% or even below 1%). These low CO₂concentrations are very difficult to separate due to the lack of drivingforce.

SHORT SUMMARY OF THE INVENTION

The present invention provides a CO₂ selective gas separation membranecomprising a gas permeable or porous support layer; and at least one gaspermeable polymer layer which is surface modified with polymer chainshaving CO₂ philic groups, wherein the gas permeable polymer layer has aspatially controlled distribution of the CO₂ philic groups on thesurface thereof. The gas permeable polymer is permeable to all gases,including CO₂. It may comprise a hydrophilic or a water vapour permeablepolymer. The CO₂ philic groups may be selected from amines, ethyleneoxide, ethers, amides or hydroxyl groups. In one embodiment, the CO₂philic groups are amines selected from the group consisting of ethylenediamine (EDA), diethylene triamine and triethylene tetramine. The gaspermeable polymer is preferably selected from the group of perfluoropolymers such aspoly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene];PTMSP (poly[1-(trimethylsilyl)-1-propyne]); PMP(poly(4-methyl-2-pentyne)); PDMS (polydimethyl siloxane) and PVA(polyvinyl alcohol). Preferably, the membrane has a brush-like structurewith spatially controlled distribution of the CO₂ philic groups.

The present invention further provides a method for preparing a CO₂selective gas separation membrane, comprising the steps of: depositingat least one gas permeable polymer layer on a porous or gas permeablesupport layer to form a dense membrane, and surface modifying the densemembrane with polymer chains having CO₂ philic groups, to obtainspatially controlled distribution of the CO₂ philic groups on thesurface thereof

In one embodiment, the surface modification of the dense membrane withCO₂ philic groups includes UV grafting. In one embodiment, in a firststep an initiator is grafted onto the at least one polymer layer by UVradiation to form grafting points, and in a second step a monomer isadded to the grafting points and polymerized by UV radiation to formgrafted polymers. If the monomer does not include CO₂ philic groups, theCO₂ philic groups may be introduced by reacting the grafted polymer withcompounds containing such CO₂ philic groups. In a preferred embodiment,the method comprises successive steps of depositing polymer layers andmodifying the surfaces thereof with CO₂ philic groups to obtain amulti-layered structure having CO₂ philic groups on top, in the middleand in bottom of the selective polymer layer leading to a controlledspatially distribution of CO₂ philic groups throughout all the membranethickness and not only on the surface.

The invention is also related to the use of the inventive gas separationmembrane for separation of CO₂ from a gas mixture.

FIGURES

FIG. 1 shows the separation principle and structure of a surfacemodified gas separation membrane (prior art).

FIG. 2 shows a three layer polymer membrane (without the support layer)having high density carrier regions with controlled spatiallydistribution comprising ultrathin layers densely packed with CO₂ philicgroups.

FIG. 3 shows a schematic view of different approaches for surfacemodification of membranes.

FIG. 4 shows the CO₂ permeance as a function of feed pressure (bar) at25° C., gas feed 10% CO₂ in N₂ fully humidified for surface modifiedPDMS/PAN membranes with GMA+EDA.

FIG. 5 shows the CO₂/N₂ selectivity as a function of feed pressure (bar)at 25° C., gas feed 10% CO₂ in N₂ fully humidified for the surfacemodified PDMS/PAN membranes with GMA+EDA.

FIG. 6 shows the CO₂ permeance as a function of feed pressure (bar) at25° C., gas feed 10% CO₂ in N₂ fully humidified for the AF2400 surfacemodified membranes with AEMA and GMA+EDA

FIG. 7 shows the CO₂/N₂ selectivity as a function of feed pressure (bar)at 25° C., gas feed 10% CO₂ in N₂ fully humidified for the AF2400surface modified membranes with AEMA and GMA+EDA.

FIG. 8 shows the CO₂ permeance as a function of temperature at 1.2 barfeed pressure, gas feed 10% CO₂ in N₂ fully humidified for the AF2400surface modified membranes with GMA+EDA.

FIG. 9 shows the CO₂/N₂ selectivity as a function of temperature at 1.2bar feed pressure, gas feed 10% CO₂ in N₂ fully humidified for theAF2400 surface modified membranes with GMA+EDA.

FIG. 10 shows CO₂ permeability versus CO₂/N₂ selectivity comparison ofdata from experiments using the inventive membrane with reported data inthe literature, see Sanders, D. F., et al., Energy-efficient polymericgas separation membranes for a sustainable future: A review. Polymer,2013. 54(18): p. 4729-4761.

FIG. 11 shows CO₂ permeability versus CO₂/CH₄ selectivity comparison ofdata from experiments using the inventive membrane with reported data inthe literature, see Sanders, D. F., et al., Energy-efficient polymericgas separation membranes for a sustainable future: A review. Polymer,2013. 54(18): p. 4729-4761.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, the surfaces of already formednon-selective/low CO₂ selective dense membranes are modified byintroducing CO₂ philic groups to become CO₂ selective. The CO₂ philicgroups on the surface of the gas permeable polymer are distributed in aspatially controlled manner. In this way, a non-selective polymer layeron a support can be modified by different methods such as UV graftingand chemical modification to become CO₂ selective.

The inventors have found that that use of certain CO₂ philic groupsprovides excellent results. Preferably, C₁-C₁₀, linear or branchedmolecules having CO₂ philic groups are used. In particular, C₁-C₆ linearor branched molecules having CO₂ philic groups or even more preferredC₁-C₃ linear or branched molecules having CO₂ philic groups are used.The CO₂ philic groups may be selected from e.g. amines, ethylene oxide,ethers, amides or hydroxyl groups. It has been found that use of longerchains molecules, larger than about C₁₀, causes crosslinking andformation of dense layers, and thus, lower CO₂ flux. In one embodiment,the CO₂ philic groups are short chain (C₁-C₃) amines such as ethylenediamine (EDA), diethylene triamine and triethylene tetramine.

The present invention provides a gas separation membrane withwell-defined geometry of the CO₂ carriers/CO₂ philic groups. Thestructure is open and the polymer chains form a brush-like structure.

The CO₂ permeability of a membrane is often expressed in Barrer. 1000Barrer represents a permeance of 2.7 m³ (STP)/(bar m² h) for 1 μm thickmembrane. In this disclosure, the term “high CO₂ permeability” meansthat the CO₂ permeability should be above 1000 Barrer. By the term “lowCO₂ selectivity” it is meant that the ratio of permeability of CO₂ withrelation to another gas (e.g. CH₄, N₂) is lower than 20.

The present invention relates to a gas separation membrane comprising atleast one CO₂ selective polymer layer being a gas permeable polymerlayer surface modified by introduction of CO₂ philic groups. The surfacemodification results in a spatially controlled distribution of CO₂philic groups on the gas permeable polymer layer.

FIG. 1 generally shows the separation principle of a surface modifiedgas separation membrane. The membrane comprises a fast-permeable region,which may be made of a polymer having high CO₂ permeability and a lowCO₂ selectivity. The polymer layer is coated on a suitable support layerfor mechanical strength. The high density carrier region comprisesultrathin layers (in FIG. 1 two layers; and in FIG. 2 three layersdensely packed with CO₂ philic groups, such as NH₂ in FIG. 1. The highconcentration of amine groups on the surface results in an increased CO₂diffusion rate and decreases the mass transfer resistance of CO₂ throughthe membrane. These layers may also partially block diffusion, from feedto permeate, of non-reactive gases such as N₂, CH₄, O₂ and H₂. The sizeof the arrows of the gases in FIG. 1, CO₂ and CH₄, illustrates theconcentration of the gases on each side of the membrane.

FIG. 3 shows two different approaches for surface modification ofmembranes depending on the type of polymer used for membranepreparation: with or without functional groups (like —OH groups forexample in polyvinyl alcohol). The membrane comprises a gas permeablepolymer layer deposited on a gas-permeable or porous support layer.

1. Grafting onto is applicable to polymers that have functional groupsto react with the end functional groups of the polymer to be attached(grafted) onto the surface (the polymer to be grafted is synthesizedseparately or purchased)2. Grafting from is applicable to polymers that do not possessfunctional groups. In this case, the monomer coupling to the membranesurface is facilitated by an initiator, which creates reactive sites onthe membrane surface (under UV radiation for example) in the first step.Then, the surface initiator initiates a graft polymerization (under UVradiation) of the added monomer and a new polymer is grown from themembrane surface in the second step.

By using any of the techniques “grafting onto” and “grafting from”described above, a brush like pattern is obtained. By using “graftingfrom” a higher density of polymer chains is achieved when compared with“grafting onto”. A CO₂ selective layer “brush like” pattern grafted ondense polymer membranes has higher permeability than a selective layerfabricated by polymer coating or by polymer precipitation onto the densemembrane surface.

The polymer brushes onto the membrane surface are created by using asequential approach. In the first step, active sites (grafting points)are created onto the membrane surface where the polymerization willbegin. The grafting points are introduced onto membrane surface bytreating the membrane with an initiator under UV radiation. In thesecond step, polymer brushes are grown onto the membrane surface byradical polymerization. The density (or the number) of the polymerchains onto the membrane surface is correlated with the density (ornumber) of the active points created in the first step as well as stericeffects between grafted polymer chains. If the attached polymer does notinclude CO₂ philic groups, these groups may be introduced in anotherstep by reacting the grafted polymer chains with C₁-C₃ alkyl compoundsbearing CO₂ philic groups, such as ethylene diamine (EDA).

The monomers suitable for growing polymer chains must fulfil the twoconditions:

1. possess functional groups not interfering with polymerization step,and

2. being capable of coupling with amines or with other CO₂ philic groupsin the third step.

The compounds bearing the CO₂ philic groups capable of coupling with thegrafted polymer chains are chosen so that the crosslinking betweenadjacent grafting polymer chains that leads to dense polymer layer isavoided or significantly lowered. The density of the CO₂ philic groupsis correlated with the molecular weight of the compounds bearing the CO₂philic groups. High densities of the CO₂ philic groups are achieved byusing low molecular weight (i.e. short chain C₁-C₁₀, linear or branchedmolecules) compounds bearing the CO₂ philic groups due to reduced stericeffects. Using shorter chain amines (C₁-C₃), such as ethylene diamine(DEA), diethylene triamine or triethylene tetramine gives the desiredbrush like structure and prevents crosslinking between the adjacentgrafted chains.

Polymer membranes are prepared by coating (dip coating and ultrasonicspray coating) on gas permeable or porous supports by using differentsolvents, viscosities of solution and drying protocols in order toobtain defect free polymer coatings having a thickness in the range from0.1 to 10 μm, preferably from 0.1 to 5 μm or from 1 to 5 μm.

Three different approaches may be used for membrane preparation:

1) Membrane formation by coating on suitable supports of highly gaspermeable polymers with low CO₂/(N₂, CH₄, O₂, H₂) selectivity (all under20) such as:

-   -   perfluoro polymers (Teflon AF2400, AF1600, etc.). Teflon AF2400:        CO₂ permeability between 3900 and 2300 Barrer.    -   PTMSP (poly[1-(trimethylsilyl)-1-propyne]) and PMP        (poly(4-methyl-2-pentyne)). Due to their poor packed polymeric        chains and glassy structure, these polymers present high free        volume leading to the highest reported CO₂ permeability; PMP:        7000 Barrer, PTMSP: 25000 Barrer.    -   Polydimethyl siloxan (PDMS), a rubbery polymer with CO₂        permeability of 2500-4000 Barrer    -   Polyvinyl alcohol, a hydrophilic polymer with good film        formation and a CO₂ permeability of ˜200-1500 Barrer        2) Surface modification of membranes with CO₂-philic, active        functional groups such as, amines (primary, secondary,        tertiary), amides, hydroxyl by various methods (wet chemistry,        UV grafting, interfacial polymerization, plasma grafting) in        order to provide gas selective membranes.        3) Ultrathin multilayer structure may be formed by consecutive        coating of highly permeable polymers on a support followed by        surface modification with CO₂ reactive groups by UV grafting:        coating-surface modification-coating-surface modification. One        and several consecutive thin layers densely packed with CO₂        philic groups can be formed.

The CO₂ philic groups will be concentrated on the surface of themembrane in extremely thin CO₂ selective layers (nanometre thickness)perpendicular to the direction of gas molecules in a brush pattern. Itis desirable to avoid certain CO₂ philic molecules, especially long,optionally branched, chain compounds (>C₁₀), that can crosslink betweeneach other leading to a very dense polymer layer that prevents theaccess of CO₂ molecules to the CO₂ philic groups, and thus, reducesmembrane permeability.

Several consecutive layers can be added on top of each other on membranesurfaces. The surface modification can be applied, both on the topmembrane layer facing the feed gas and/or the bottom layer of membranes,facing the permeate side. In addition, several layers in the “middle” ofthe membrane may be formed by a consecutive deposition of non-selectivepolymer-surface modification, followed by coating of a secondnon-selective layer followed by surface modification, and so on.

Various gas permeable or porous supports may be used. They may be madeof materials such as polysulfone (PSF), polyethersulfone (PES) polyamide(PA), polyimide (PI), polyvinyl difluoride (PVDF), polyacrylonitrile(PAN) or cellulose acetate (CA). The thickness of this support layer mayvary from 10 to 250 μm. Preferably, the pore size of the porous layer isfrom 0.0001 μm to 1 μm.

A dense layer made of gas permeable polymers having high gaspermeability can also be used as mechanical support under the CO₂selective layer. This layer can be supported as well on an additionalporous layer underneath, and is then called a gutter layer. Thethickness of such a dense layer may vary, from about 0.1 to 1 μm (whenadditional porous support is used) up to around 200 μm (without poroussupport). Examples of suitable high gas permeable polymers arepolydimethylsiloxane (PDMS), poly(l-trimethylsilyl-1-propyne) (PTMSP),polymethylpentene (PMP) or amorphous fluoropolymers such as4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene.

The gas separation membrane of the invention may further comprise aprotective layer made of high gas and water vapour permeability materialcoated on top of the CO₂ selective polymer layer. Suitable materials forthe protective layer are polydimethylsiloxane (PDMS),poly(l-trimethylsilyl-1-propyne) (PTMSP), polymethylpentene (PMP) oramorphous fluoropolymers such aspoly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene].

The CO₂ selective polymer layer may have a single or a multilayerstructure. The multilayer structure may comprise several surfacemodified polymer layers together with one or more non-selective polymerlayer(s) for protection of the surface modified polymer layers.

In the present invention, modification by UV-grafting has shownexcellent results. Surface modification of polymers by UV grafting is ageneric approach that to a large extent will be independent of thechemical composition of the polymer. A variety of new functional groupscan be introduced to surfaces, for instance amino groups, by applyingvarious grafting techniques and procedures. The preferred strategy forUV-grafting is based on a sequential UV-grafting procedure performed intwosteps. Concerning specific membranes, polydimethylsiloxane (PDMS) andpoly[1-(trimethylsilyl)-1-propyne] (PTMSP) will preferably be modifiedby UV grafting, but polydimethylsiloxane could also be modified byhydrolysis. Polyvinyl alcohol could alternatively be chemicallymodified. Several methods will be used for membranes structurecharacterization: SEM pictures, FT-IR, AFM.

UV grafting of membranes is a general method, which can be used tointroduce reactive groups or a functional layer on polymeric membraneswhen the polymer lacks functional groups that can be used for furthermodification by chemical coupling. The advantage of using UV graftingfor polymeric membranes resides in its simplicity, cleanness and easyscalability.

Polymers such as PDMS and PTMSP do not possess functional groups.UV-grafting technique can be used to introduce functional groups ontothe surface of PDMS and PTMSP membranes that can be further modified bychemical coupling.

EXPERIMENTS

Surface Modification Methods for all Membranes

A sequential 3 steps method was used to modify membranes by UV grafting.Three monomers have been tested: Glycidyl methacrylate (GMA), aminoethyl methacrylate (AEMA) and diethyl aminoethyl acrylate (DEAEA). Aftergrafting of poly-GMA onto membrane surface, amine groups are introducedby coupling with ethylene diamine (EDA).

1 Initiator Grafting onto Membrane Surface

An initiator, in this case benzophenone, was grafted onto a membranesurface by UV radiation. The excess of initiator was then removed toprevent unwanted side reactions. The density of the grafting points isdepending on the concentration of the initiator.

2. Polymerization Step

The monomer was added and the polymerization promoted by exposing themembrane covered by monomer solution to UV radiation. The polymerizationideally starts in the grafting points on the membrane surface. Thepolymer chain lengths depend on the monomer concentration and the UVexposure time. The two-step approach has the advantage that it reducesundesired side reactions.

Poly amino ethyl methacrylate- and polyethylene diamine modifiedmembranes preparation are prepared by steps 1 and 2, while the membranesmodified with glycidyl methacrylate (GMA) need an additional step tointroduce the CO₂-philic groups

3. Introduction of CO₂-Philic Groups

Poly glycidyl methacrylate (GMA) modified membranes were further reactedwith polyethylene imine (PEI) or ethylene diamine (EDA) to introduce theCO₂-philic groups: The membranes grafted with GMA were coated with:

PEI in borate buffer pH 9.3

-   -   20-50% EDA in water or borate buffer at pH 9.3.

EXAMPLES and RESULTS Example 1. Polyvinyl Alcohol (PVA) Based MembranesSurface Modified

Polyvinyl alcohol (PVA) membranes were prepared by solution casting onporous support of polysulfone (PSF) with 50 000 MWCO. Commercial PVA onpolyacrylonitrile (PAN) support membranes were used as well for surfacemodification with amines. They were modified according to steps 1-3mentioned above. The tests were performed with mixed gases, 10% CO₂ inN₂ fully humidified at 25° C., and feed pressure from 1.2 to 5 bar,absolute pressure and the results are presented in Table 1.

Table 1 shows the comparative results of reference membrane (withoutsurface modification) and surface modified membranes.

TABLE 1 CO₂ permeability/permeance and CO₂/N₂ selectivity of PVAmembranes surface modified CO₂ CO₂ permeance permeability m³ (STP)/CO₂/N2 Membrane (Barrer) (m² bar h) selectivity PVA/PSF supportReference 246 0.09 11 PVA/PAN support grafted Not 0.02 57 with GMA + EDAdetermined Obs. CO₂ permeability is equal to CO₂ permeance multipliedwith membrane thickness for a given membrane. 1000 Barrer represents apermeance of 2.7 m³ (STP)/(bar m²h) for 1 micrometre thick membrane.

As can be seen, the CO₂/N₂ selectivity increased several times forsurface modifications performed with amines (EDA) compared to referencemembranes.

Example 2. PDMS Based Membranes Surface Modified with Amine Groups

The results were obtained by using commercial PDMS on PAN support whichwere modified with glycidyl methacrylate (GMA) first followed byreaction with 20% ethylene diamine (EDA) in aqueous solution accordingto method steps 1-3 described above. The results, CO₂ permeance andCO₂/N₂ selectivity as function of feed pressure, are shown in FIG. 4 andFIG. 5, respectively.

The surface modification reduced the CO₂ permeance, but increased theCO₂/N₂ selectivity 3-5 times compared with the reference membrane due toamine groups grafted on surface.

Example 3. PTMSP Based Membranes Surface Modified with Amine Groups

Self-standing membranes of poly(l-trimethylsilyl-1-propyne) PTMSP, wereprepared by solvent casting from cyclohexane. The membranes weremodified according to method steps 1-3 described above. The results arepresent in Table 2.

TABLE 2 CO₂ permeability/permeance and CO₂/N₂ selectivity of PTMSPmembranes surface modified CO₂ CO₂ permeance permeability m³ (STP)/CO₂/N₂ Membrane (Barrer) (m² bar h) selectivity PTMSP reference 228402.5 6 PTMSP + AEMA 13650 0.75 11 PTMSP + GMA 18154 0.49 7 ReferencePTMSP + GMA + EDA 14726 0.4 13 PTMSP + DAEA 2940 0.16 16 Obs. CO₂permeability is equal to CO₂ permeance multiplied with membranethickness for a given membrane. 1000 Barrer represents a permeance of2.7 m³ (STP)/(bar m²h) for 1 micrometre thick membrane.

All surface modification methods reduced the CO₂ permeance more or lesscompared to the reference membrane, however, the CO₂/N₂ selectivity dueto amine groups grafted on surface was doubled.

Example 4. Perfluoro Membranes (AF2400) Based Membranes Surface Modifiedwith Amine Groups

Exceptional results were obtained when using the surface modificationapproach with a surface modified perfluoro membrane (AF2400) by UVirradiation using method steps 1-3 described above, and amines AEMA andGMA+EDA. For a 50 μm membrane, a CO₂ permeability of 1900 Barrer (0.1 m³(STP)/(bar m² h) and a CO₂/N₂ selectivity over 500 at 55° C. wereobtained, using humidified 10% CO₂ in N₂.

FIG. 6 and FIG. 7 show the dependence on feed pressure of CO₂ permeanceand CO₂/N₂ selectivity, respectively. It is clearly observed that theCO₂ permeance decreased, however, a tremendous CO₂/N₂ selectivity isachieved, especially for membranes modified with GMA+EDA.

FIG. 8 and FIG. 9 show the temperature dependence of CO₂ permeance andCO₂/N₂ selectivity, respectively, for AF2400 membranes modified withGMA+EDA. The CO₂ permeance increases and CO₂/N₂ selectivity slightlydecreases from 25 to 55° C. (flue gas temperature). This is probably dueto increase of diffusion coefficients with temperature for both CO₂ andN₂ gases.

Results were well above the best polymeric membranes previously reportedin the literature for CO₂/N₂ separation (Sanders, D. F., et al.,Energy-efficient polymeric gas separation membranes for a sustainablefuture: A review. Polymer, 2013. 54(18): p. 4729-4761). The result at1.2 bar feed pressure is plotted as

for an AF2400 membrane modified with GMA+EDA in FIG. 10 and comparedwith literature data.

Example 5. Perfluoro Membranes (AF2400) Based Membranes Surface Modifiedwith Amine Groups for CO₂/CH₄ Separation (Natural Gas)

The test was performed with mixed gases, 10% CO₂ in CH₄, fullyhumidified at 25° C. (similar conditions to natural gas) at 2 and 5 barfeed pressure. The results obtained with a 50 μm thick membrane, areshown in table 3 below.

TABLE 3 CO₂ permeability/permeance and CO₂/CH₄ selectivity of AF2400membranes surface modified with GMA + EDA CO₂ CO₂ permeance permeabilitym³ (STP)/ CO₂/CH₄ Membrane/pressure (Barrer) (m² bar h) selectivityAF2400 Reference/ 2135 0.48 5 2 bar AF2400 + GMA + 1332 0.073 548 EDA/2bar AF2400 + GMA + 804 0.044 249 EDA/5 bar Obs. CO₂ permeability isequal to CO₂ permeance multiplied with membrane thickness for a givenmembrane. 1000 Barrer represents a permeance of 2.7 m³ (STP)/(bar m²h)for 1 micrometre thick membrane.

The results obtained for the surface modified membranes were well abovethe best polymeric membranes previously reported in the literature forCO₂/CH₄ separation (Sanders, D. F., et al., Energy-efficient polymericgas separation membranes for a sustainable future: A review. Polymer,2013. 54(18): p. 4729-4761). The result for the membrane AF2400+GMA+EDAat 5 bar is plotted as a star and compared with literature data in FIG.11.

1. A CO₂ selective dense gas separation membrane comprising: a gas permeable or porous support layer; and at least one dense gas permeable polymer layer deposited on the gas permeable or porous support layer, the dense gas permeable polymer layer is surface modified with polymer chains having CO₂ philic groups, and constitutes a CO₂ selective layer in nanometre thickness having an open, brush-like structure of the polymer chains, providing spatially controlled distribution of the CO₂ philic groups on a surface of the gas permeable polymer layer.
 2. The gas separation membrane according to claim 1, wherein the CO₂ philic groups are functional groups of C₁-C₁₀ linear or branched molecules.
 3. The gas separation membrane according to claim 1, wherein the polymer chains having CO₂ philic groups are grafted by UV radiation from grafting points on the surface of the CO₂ permeable polymer layer.
 4. The gas separation membrane according to claim 1, wherein the gas permeable polymer layer is a hydrophilic or a water vapour permeable polymer.
 5. The gas separation membrane according to claim 1, wherein the CO₂ philic groups are selected from amines, ethylene oxide, ethers, amides or hydroxyl groups.
 6. The gas separation membrane according to claim 5, wherein the CO₂ philic groups are amines selected from ethylene diamine (EDA), diethylenetriamine or triethylenetetramine.
 7. The gas separation membrane according to claim 1, wherein the gas permeable polymer layer comprises a perfluoro polymer.
 8. A method for preparing a CO₂ selective gas separation membrane, comprising the steps of: depositing at least one gas permeable polymer layer on a porous or gas permeable support layer to form a dense membrane, surface modifying the dense membrane by the sequential steps: creating grafting points onto the membrane surface by treating the surface with an initiator; removing the unreacted initiator and adding a monomer solution to the membrane surface; creating polymer chains starting from the grafting points; wherein the monomer solution either forms polymer chains with or without CO₂ philic groups, wherein the polymer chains without CO₂ philic groups, can be functionalized with CO₂ philic groups, forming a CO₂ selective layer in nanometre thickness having an open, brush-like structure of the polymer chains, providing spatially controlled distribution of the CO₂ philic groups on the surface of the gas permeable polymer layer.
 9. The method according to claim 8, wherein the grafting points are created by grafting an initiator onto a membrane surface by UV radiation.
 10. The method according to claim 9, wherein the monomer solution is polymerized by UV radiation to form grafted polymer chains starting from the grafting points.
 11. The method according to claim 8, wherein the monomer solution forms polymer chains with CO₂ philic groups.
 12. The method according to claim 8, wherein the monomer solution forms polymer chains without CO₂ philic groups, the polymer chains are functionalized by reacting the grafted polymer chains with molecules containing CO₂ philic groups, wherein the molecules containing CO₂ philic groups are selected from C₁-C₁₀, linear or branched molecules having CO₂ philic groups.
 13. The method according to claim 8, comprising successive steps of depositing polymer layers and modifying the surfaces thereof with CO₂ philic groups to obtain a multi-layered structure having CO₂ philic groups attached to the CO₂ permeable polymer layer on top, in the middle and in bottom of the polymer layers.
 14. A method of separating CO₂ from a gas mixture comprising: contacting the gas mixture with a CO₂ selective dense gas separation membrane, the gas separation membrane including: a gas permeable or porous support layer; and at least one dense gas permeable polymer layer deposited on the gas permeable or porous support layer, the dense gas permeable polymer layer is surface modified with polymer chains having CO₂ philic groups, and constitutes a CO₂ selective layer in nanometre thickness having an open, brush-like structure of the polymer chains, providing spatially controlled distribution of the CO₂ philic groups on the surface of the gas permeable polymer layer; and removing CO₂ molecules from the gas separation membrane.
 15. The gas separation membrane according to claim 7, wherein the perfluoro polymer comprises poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene] or PDMS (polydimethyl siloxane). 